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Wildlife Research, 2009, 36, 81–97
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Use of artificial tree hollows by Australian birds and bats
Ross L. Goldingay A,B and Jane R. Stevens A
A
School of Environmental Science and Management, Southern Cross University,
PO Box 157, Lismore, NSW 2480, Australia.
B
Corresponding author. Email: [email protected]
Abstract. Artificial tree hollows (nest or roost boxes) may be of considerable importance to the conservation and
management of Australian hollow-using birds and microbats. This is suggested by recognition that the rate of collapse of
hollow-bearing trees may exceed replacement in some landscapes. We review the published literature to synthesise current
information on the use of artificial hollows by Australian birds and bats, and to provide guidance to future research and
management. The use of artificial hollows has been documented in some detail for 15 native bird and eight microbat species.
A range of hollow designs has been employed but there is a limited understanding of favoured designs. General designs
(e.g. front-entry plywood boxes) have been used extensively by some species and should continue to be used until more
effective designs are identified. Species tend to use artificial hollows that have entrance sizes just sufficient for their body size,
and this should guide hollow design. Competitive interactions with a range of non-target species (native and non-native) may
have a pronounced influence on artificial hollow use and must be considered in any management program involving artificial
hollows. We highlight some design elements that may reduce interference by non-target species. Temperature inside artificial
hollows may have a particular influence on their use by bats due to the role of microclimate in bat thermoregulation. Trials are
needed to investigate this factor and to inform general approaches to positioning of artificial hollows. Several distinct
management uses exist for artificial hollows, including assisting threatened species recovery, e.g. Kangaroo Island glossy
black-cockatoo (Calyptorhynchus lathami halmaturinus) and orange-bellied parrot (Neophema chrysogaster). Artificial
hollows offer an interim solution to hollow shortage but their full potential will only be realised when preferences for different
designs are better understood. This will require a commitment to monitoring and should be conducted in an adaptive
management context. Increased knowledge of the use of artificial hollows by Australian birds and bats should be of global
relevance to the management of hollow-using species because it provides an independent test of ideas and strengthens any
generalisations.
Introduction
Species of wildlife throughout the world use tree hollows for
protection when inactive and as breeding sites (e.g. Saunders et al.
1982; Mackowski 1984; Newton 1994; Nowak 1999; Kunz and
Lumsden 2003; Zinner et al. 2003). In Australia, as many as 300
native vertebrate species (birds, bats, arboreal marsupials, reptiles
and frogs) use tree hollows (Gibbons and Lindenmayer 2002).
This corresponds to a far greater proportion of wildlife species
dependent on tree hollows for survival compared to that on other
continents (see Saunders et al. 1982; Newton 1994; Gibbons and
Lindenmayer 2002). This prevalence of hollow dependence
among Australian wildlife should result in land managers and
researchers leading the way internationally in understanding the
hollow requirements of species and devising effective
management programs for them. At present, this does not
appear to be the case with widespread concern that the
abundance of tree hollows has declined within many
landscapes in Australia (Joseph et al. 1991; Bennett et al.
1994; Mawson and Long 1994; Lindenmayer et al. 1997; Pell
and Tidemann 1997a; Gibbons et al. 2002; Heinsohn et al. 2003;
Eyre 2005; Harper et al. 2005a; Courtney and Debus 2006).
As a consequence, many hollow-dependent species are now listed
CSIRO 2009
as threatened (Gibbons and Lindenmayer 2002). Management
programs in response to this appear to be in their infancy.
Artificial hollows (nest and roost boxes) can potentially
simulate the tree hollow environment for many hollow-using
species (Menkhorst 1984; Boyd and Stebbings 1989; Petty et al.
1994; Smith and Agnew 2002; Harley 2004; Beyer and
Goldingay 2006; Goldingay et al. 2007) and this is well
recognised in Europe and North America (Stebbings and
Walsh 1991; Newton 1994). Indeed, understanding factors that
may influence the use of artificial hollows is an active area of
research in Europe and North America for birds (e.g. Pogue and
Schnell 1994; Ardia et al. 2006) and bats (e.g. Kerth et al. 2001;
Flaquer et al. 2006). Artificial hollows have long been recognised
as an important research and management tool for Australian
arboreal marsupials (Menkhorst 1984; Beyer and Goldingay
2006). In contrast, the use of artificial hollows by Australian
birds is poorly documented in the published literature despite the
deployment of artificial hollows as a management tool for over a
decade (e.g. Olsen 1996; Mooney and Pedler 2005). There is
increasing evidence that roost boxes will be used by Australian
microbats (Golding 1979; Irvine and Bender 1995; Ward 2000;
Smith and Agnew 2002) but the extent to which this can result in
10.1071/WR08064
1035-3712/09/020081
Wildlife Research
valuable research and management applications is not well
understood.
This paper reviews published information on the use of
artificial hollows by Australian birds and bats. The large
number of hollow-using species in Australia means that any
increase in knowledge and understanding can benefit the
management of a significant component of biodiversity.
Specifically we aim to: (i) synthesise available published
information on the use of artificial hollows; (ii) examine
different applications of artificial hollows; and (iii) identify
gaps in our knowledge so as to stimulate future research. The
present review complements one by Beyer and Goldingay (2006)
concerning the use of artificial hollows by Australian arboreal
marsupials. We make extensive reference to studies in the
northern hemisphere because there is a substantial literature
for that area that can greatly inform our understanding of the
requirements and behaviour of Australian species. Furthermore,
an increased understanding of Australian species can provide
independent tests of various hypotheses about the use of artificial
hollows, which will allow greater generalisation for any
hollow-using birds and bats.
Use of artificial hollows by Australian birds and microbats
To review the literature on the use of artificial hollows by
Australian birds and bats, we conducted searches of the Web
of Science as well as the Australian journals Australian Journal
of Zoology, Emu and Wildlife Research, using webpage search
tools and keywords relating to artificial hollows. The journal
Australian Mammalogy was searched manually. Theses were
only included when commonly referred to by published studies.
Some studies and most books on this topic have documented
species observed using artificial hollows but have provided few
details of such use. We restrict our attention to studies that
describe more than one observation of nest and roost box use
by birds or bats, and in which some details of the artificial hollows
used were described. Where possible we have collated data on all
species referred to in studies, regardless of whether one species or
wildlife group was targeted.
Only 27 studies have been published during the last 32 years
on the use of nest boxes by birds, with 14 published during
1996–2000 (Fig. 1). This included 15 native and two introduced
species. Nest box use has been described for seven parrot species,
three passerines, one nocturnal bird and two waterfowl species.
The use of artificial hollows by microbats has fared much more
poorly with just five studies covering eight species published in
the last 32 years (Fig. 1).
The most commonly encountered species in artificial
hollows, in terms of the number of individuals or boxes
occupied and the number of studies in which they featured,
were the crimson rosella (Platycercus elegans), chestnut teal
(Anas castanea), common myna (Acridotheres tristis), Gould’s
wattled bat (Chalinolobus gouldi) and the large forest bat
(Vespadelus darlingtoni) (Table 1). Welcome swallows
(Hirundo neoxena) used at least 800 plastic drum nest boxes
in Western Australia but no further details were provided
(Norman and Riggert 1977). The chestnut teal (Anas castanea)
accounted for 363 boxes occupied in Victoria (Norman and
Riggert 1977). This and one other study (Briggs 1991) are the
R. L. Goldingay and J. Stevens
16
Number of studies
82
14
Microbats
12
Birds-Management
10
Birds-Research
8
6
4
2
0
1976–80
1981–85
1986–90
1991–95 1996–00
2001–05
2006+
Year of publication
Fig. 1. The number of studies on the use of artificial hollows by birds and
microbats published in 5-year periods since 1976.
only ones that have targeted waterfowl despite the use of tree
hollows for nesting by 10 Australian species (Saunders et al.
1982; Marchant and Higgins 1990) and the common use of nest
boxes to study waterfowl in the USA (see Eadie et al. 1998; Evans
et al. 2002). Sample sizes in Australian studies exceeded 50
(either boxes occupied or individuals encountered) for just six
species (Table 1), though some studies did not document the
number of individuals. The endangered orange-bellied parrot
(Neophema chrysogaster) and Kangaroo Island glossy blackcockatoo (Calyptorhynchus lathami halmaturinus) have used
nest boxes repeatedly but the details of this are not well
documented (Mooney and Pedler 2005; Orange-bellied Parrot
Recovery Team 2006).
Factors that affect the use of artificial hollows
Several factors will influence the use of nest and roost boxes.
Of particular importance are hollow design and placement,
natural hollow availability, and competition with other species.
Breeding patterns will produce seasonal patterns of use, and these
may differ across taxa. Temperature is also a key factor and may
produce a seasonal response in the timing of use. These factors
have not been well studied and represent significant gaps in our
understanding of artificial hollow use.
Hollow design and placement
The main design elements that may influence the use of artificial
hollows are entrance size, hollow volume, hollow depth below
entrance and wall thickness. A great amount of general literature
is available recommending detailed box designs for birds and
bats (McCulloch and Thomas 1986; Stebbings and Walsh 1991;
Grant 1997; Franks and Franks 2003). However, many
recommendations appear to have been developed in an ad hoc
way. Understanding whether species show preferences for
particular design elements and placement positions is central
to all research and management applications of artificial hollows.
Despite this only a few published studies have provided a choice
of hollow types or design elements for birds and bats in Australia
(see section Research applications).
In the absence of preference studies (e.g. Lumsden 1989;
Radunzel et al. 1997) it is likely that future studies will simply
follow designs used elsewhere and be unable to state whether low
–
–
XX
X
–
X
X
X
X
–
–
24
24
Volume (m3)
Small (<0.005)
Medium (0.005–0.03)
Large (>0.03)
Height above ground
<2 m
2–3.9 m
4–6 m
>6 m
Aspect
North
South
East
West
Sample sizes
Individuals
Boxes occupied
3, 4,
5, 8
–
XX
–
Entrance location
Slit below
Front/top
Spout
References
–
X
XX
X
X
24
Australian
owlet-nightjar
Entrance diameter
<3 cm
3–4.9 cm
5–8 cm
8.1–12
>12
Body size – birds (cm),
bats (g)
Box variable
10
>2
>2
–
–
–
–
–
–
–
–
–
–
X
–
–
–
–
–
–
–
–
3, 4,
5, 8
21
21
X
X
–
–
–
–
X
X
–
–
XX
–
XX
–
–
–
XX
–
–
15
Norfolk Island
boobook owl
White-throated
treecreeper
30
Grey shrike-thrush
5
–
3
X
X
–
–
X
–
X
X
–
–
X
–
X
–
–
–
–
–
X
24
Striated pardalote
2
4
4
–
–
–
–
–
XX
–
–
XX
–
–
–
XX
–
–
XX
–
–
–
12
Crimson rosella
Eastern rosella
–
8
X
–
X
–
X
–
X
–
–
X
–
–
X
–
–
–
X
X
–
30
–
6
X
–
–
–
–
–
X
–
–
X
–
–
X
–
–
–
–
X
–
28
Rainbow lorikeet
3, 4, 8, 12, 17
5,
14, 15
12, 13
>85
>95
X
X
–
–
X
–
XX
X
–
XX
XX
–
XX
XX
–
–
XX
X
–
35
Red-rumped
parrot
8
–
4
–
–
X
–
–
–
–
–
–
X
–
–
X
–
–
–
–
–
–
27
Glossy
black-cockatoo
11
–
41
–
–
–
–
–
–
–
XX
–
–
XX
–
XX
–
–
–
–
–
XX
48
Red-tailed
black-cockatoo
9
5
5
–
–
–
–
–
–
–
X
–
–
XX
–
–
XX
–
–
–
–
XX
63
Galah
8, 18
–
2
X
–
X
–
–
–
X
–
–
X
–
–
X
–
–
–
–
X
–
36
Orange-bellied
parrot
19
>30
28
–
–
–
–
–
–
–
–
–
X
–
–
X
–
–
X
–
–
–
21
Chestnut teal
1
>360
>360
–
–
–
–
XX
XX
–
–
–
XX
XX
–
X
–
–
–
–
–
XX
48
Turquoise parrot
6
2
2
–
–
–
–
X
X
–
–
–
X
–
–
–
X
–
–
X
–
–
20
Common starling
Common myna
–
81
XX
XX
XX
–
–
–
XX
–
–
XX
–
–
XX
–
–
–
XX
X
–
25
>250
30
X
X
–
X
–
X
X
X
X
XX
X
XX
X
–
XX
–
X
–
–
12
Gould’s wattled
bat
12, 14, 12, 15, 3, 4, 7
15
18
–
12
–
X
–
–
–
–
X
–
–
XX
–
–
X
–
–
–
X
–
–
22
Gould’s
long-eared bat
17,
20
50
24
X
X
X
X
–
X
X
–
X
X
–
XX
–
–
XX
–
–
–
–
9
Eastern false
pipistrelle
Lesser
long-eared bat
56
–
–
–
–
–
–
–
X
X
–
X
X
–
X
–
X
–
X
–
–
7
Large forest bat
>100
10
–
–
–
–
–
–
X
X
X
X
X
X
X
–
X
–
–
–
–
7
>2
10
–
–
–
–
–
–
X
X
X
X
X
X
X
–
X
–
–
–
–
5
Southern forest bat
3, 4, 3, 4, 7, 16 7, 16
16 7, 16
–
3
–
–
–
–
–
–
X
X
–
–
X
X
X
–
X
–
–
–
–
21
Chocolate
wattled bat
7
>1
–
–
–
–
–
–
–
X
–
X
X
–
X
X
–
X
–
–
–
–
8
7
>5
–
–
–
–
–
–
–
X
–
X
X
–
X
–
–
X
–
–
–
–
38
White-striped
freetail bat
Table 1. Characteristics of artificial hollows used by different species of bird and bat
Frequency of use of variable categories: XX = frequently used (>20% of boxes used at a site or in a study); X = known to have been used; – has not been used or absent from studies. Not all variable categories were
present where species used nest boxes. Sample sizes have been pooled across studies. Some studies may not give precise data so ‘>’ is used to indicate the true value exceeds that shown. Species body sizes,
Simpson and Day (1993), Churchill (1998). References: 1, Norman and Riggert (1977); 2, Milledge (1978); 3, Golding (1979); 4, Calder et al. (1983); 5, Menkhorst (1984); 6, Quin and Baker-Gabb (1993);
7, Irvine and Bender (1995), R. Bender and R. Irvine (unpubl. data), R. Bender (unpubl. data); 8, Trainor (1995a, 1995b); 9, Emison (1996); 10, Olsen (1996); 11, Pedler (1996); 12, Pell and Tidemann
(1997a, 1997b); 13, Krebs (1998, 1999), Krebs et al. (1999); 14, Gleeson (1999); 15, Homan (1999); 16, Ward (2000); 17, Smith and Agnew (2002); 18, Harper et al. (2005b); 19, Orange-bellied Parrot
Recovery Team (2006); 20, Goldingay (2007)
Use of artificial tree hollows by Australian birds and bats
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83
84
Wildlife Research
usage was due to the design or local habitat factors. Studies
reporting use of artificial hollows by bats are a good example of
this. Golding (1979) used 22 sawn-timber boxes and 143 adapted
log hollows. Both designs were of similar volume and had a
6.5 cm diameter circular front entrance. He reported use of 23
boxes by 260 bats of five species but did not report whether one
design was favoured. Menkhorst (1984) reported no use by bats of
240 front-entry boxes with circular entrances of 5–15 cm
diameter. Irvine and Bender (1995) recorded 34 bats in 5 of 10
boxes with a slit-entry at the base. Ward (2000) used a small box
with a slit-entry under the lid and reported 73 captures of four
species of bats. Smith and Agnew (2002) found that 17 of 48
wedge-shaped boxes with a basal slit-entry were used by bats at
two sites. It is impossible to determine from these studies
spanning a broad latitudinal range whether one design or
design element (e.g. slit v. circular entry) might be favoured.
Given the influence of temperature on roost selection by bats (see
below), it is also likely that box temperature has played a role in
these results.
We have summarised information on the range in box design
elements used by bird and bat species (Table 1), which provides a
broad indication of preference in the absence of direct testing. The
materials that boxes are made from have varied substantially
among studies. Hollows have been made from wooden
ammunition boxes and plastic drums for use by waterfowl
(Norman and Riggert 1977), salvaged tree hollows placed in
trees for turquoise parrots (Quin and Baker-Gabb 1993) or in trees
and on powerpoles for red-tailed black-cockatoos
(Calyptorhynchus banksii graptogyne) (Emison 1996; Fig. 2),
polyvinyl chloride (PVC) pipe for use by glossy black-cockatoos
on Kangaroo Island (Pedler 1996) (Fig. 3), and plywood boxes for
eastern rosellas (Fig. 4) and crimson rosellas (Krebs 1998) and for
bats (Fig. 5). In Britain, plastic drums have been used extensively
by barn owls (Petty et al. 1994). Bat boxes have been made from
marine ply (Smith and Agnew 2002) or from pine (Irvine and
Bender 1995).
No detailed studies have been conducted in Australia to
examine the influence of roost box design on bat occupancy.
A variety of bat box designs have been used overseas, and often in
the context of providing alternative roost sites to bats displaced
from roof spaces (Neilson and Fenton 1994). Brittingham and
Williams (2000) tested a vertical and a horizontal box of the same
dimensions and found a preference for the horizontal box. They
also suggested that boxes needed to experience high internal
temperatures. Flaquer et al. (2006) compared single- and doublecompartment bat boxes installed on houses, posts and trees. They
found greater use of the double-compartment box, and boxes
attached to houses and posts, and recorded the highest occupancy
by bats of any studies using bat boxes. Their single-compartment
box was similar to that used by Irvine and Bender (1995) but with
a 1.5-cm rather than a 3-cm slit opening at the base.
Entrance size is a key design element, possibly enabling boxes
to be designed to target species and it is one element that is easily
varied. Most species generally prefer a minimum entrance size to
allow access (i.e. one close to body width) but avoid predation by
larger species. Menkhorst (1984) found that crimson rosellas
preferred one entrance size (8 cm) compared to three others.
Several studies found that the Australian owlet-nightjar
(Aegotheles cristatus) occupied a range of entrance diameters,
R. L. Goldingay and J. Stevens
(a)
(b)
Fig. 2. (a) An artificial hollow installed for red-tailed black-cockatoos in
western Victoria. Metal flashing was attached near the base of the pole to
exclude access by brushtail possums. (b) This artificial hollow consisted of a
salvaged tree hollow attached to a powerpole at a height of approximately
12 m. Photos: R. Goldingay.
though it was more frequently encountered in boxes with
entrances of 5–8 cm diameter (Table 1) (Fig. 6). Entrance size
may be important to exclude some predatory species. Krebs
(1998) implicated pied currawongs (Strepera graculina) in
high levels of predation on crimson rosella chicks that ceased
when she used a 10 cm-long PVC tube to create a spout entrance.
Most studies (Table 1) have found that bats generally use small
(<3 cm) slit openings located at the bottom (Smith and Agnew
2002; Goldingay et al. 2007; R. Bender, unpubl. data) or top
(Ward 2000) of boxes. Large entrances (suitable for larger
mammals) are thought to deter bats from nest boxes
(Menkhorst 1984), though Golding (1979) reported the use of
23 boxes with a 6.5 cm diameter circular opening by bats. Boxes
with slit entrances >20 mm were found occupied by competing
larger mammals such as the sugar glider (Petaurus breviceps),
introduced black rat (Rattus rattus) and common ring-tailed
possum (Pseudocheirus peregrinus) (R. Bender and R. Irvine,
unpubl. data) or by feathertail gliders (Acrobates pygmaeus),
sugar gliders and squirrel gliders (Petaurus norfolcensis)
(Smith and Agnew 2002; Goldingay et al. 2007).
Use of artificial tree hollows by Australian birds and bats
(a)
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85
(b)
Fig. 3. (a) A polyvinyl chloride artificial hollow installed for glossy black-cockatoos on Kangaroo Island. (b) The same design
of artificial hollow (~90 cm tall, 30 cm diameter; entrance 16 20 cm) installed for red-tailed black-cockatoos in western Victoria
on a powerpole. Photos: (a) E. Sobey; (b) R. Goldingay.
Fig. 4. Nestling eastern rosellas in a plywood nest box. Photo: R. Goldingay.
Volume and depth are thought to influence the suitability of
artificial hollows to birds and bats (Trainor 1995a) but
information on this is limited. These parameters of natural
hollows influence selection of hollow sites (Saunders et al.
1982; Gibbons et al. 2002), and are known to affect the
breeding success of birds (Newton 1994). Nest box depth may
provide security from predators (Trainor 1995a).
The insulating ability of nest boxes is determined by the type
and thickness of materials used. Insulation is thought to affect nest
box use (Calder et al. 1983; Menkhorst 1984), leading to natural
hollows being favoured when available. Menkhorst (1984)
postulated this to explain the relative absence of nest box use
by the Australian owlet-nightjar and white-throated tree-creeper
(Cormobates leucophaea) at one site where natural hollows were
abundant compared to another with low abundance where boxes
were used. Trainor (1995a) hypothesised that low insulating
properties may affect the breeding success of birds. Common
mynas showed no preference for insulating properties (25-mm
rough-sawn pine or 12-mm plywood) in one study (Harper et al.
2005b). The insulating ability of nest boxes may be significant to
the requirements of bats, which are thought to use hollows that
facilitate thermoregulation (Gibbons and Lindenmayer 2002;
Lourenço and Palmeirim 2004). R. Bender (unpubl. data)
suggested no greater use of 10 roost boxes with a thick wall
(45 mm) designed for winter roosting by bats compared to other
boxes (19 mm), though the extent to which this changed the
microclimate inside these boxes is unknown.
Aspect is likely to influence temperature changes within
nest and roost boxes (Stebbings and Walsh 1991; Brittingham
and Williams 2000; Ardia et al. 2006). Its influence may differ
between birds and bats due to the thermoregulatory requirements
of bats (see below). No preference for nest box aspect has been
detected in any Australian study (Table 1), though few species
have been assessed and sample sizes have been small (Calder et al.
1983; Menkhorst 1984; Smith and Agnew 2002). Furthermore,
the actual position of boxes on trees may not have provided an
adequate test of aspect (Menkhorst 1984). Stebbings and Walsh
86
Wildlife Research
R. L. Goldingay and J. Stevens
(a)
(b)
Fig. 5. (a) A wedge-shaped artificial hollow for bats (26 20 18 cm; 1.5-cm slit-entry). (b) Gould’s wattled bats in a wedgeshaped bat box. Photos: (a) R. Goldingay; (b) M. Grimson.
(a)
(b)
Fig. 6. (a) An artificial hollow designed for rosellas (45 20 15 cm; 6.5-cm entrance). No research has addressed the need for
perches on these hollows. (b) An owlet-nightjar roosting in this artificial hollow. Photos: R. Goldingay.
(1991) recommended choosing aspects for bat boxes that allow
the sun to fall directly on the box for part of the day, as well as
providing boxes with various aspects in case overheating should
occur. Preference trials are needed to resolve this issue. Placement
should also consider the surrounding habitat because some
species may be influenced by habitat variables in their choice
of suitable nest boxes (e.g. Willner et al. 1983; Pogue and
Schnell 1994).
Few studies have tested whether bird or bat species show a
preference for the height at which boxes are placed in trees. Most
studies placed boxes at heights of 2–6 m, presumably because
artificial hollows placed at low heights will be quicker and safer to
monitor. These relatively low heights may have limited the use of
boxes but for the species detected there is little evidence as yet that
greater heights will be preferred (Table 1). Stebbings and Walsh
(1991) suggested that some bat species in Great Britain prefer
boxes to be at least 5 m high. The height at which boxes are placed
may influence rates of predation (Milledge 1978; Menkhorst
1984) but this remains to be tested. Lumsden et al. (2002a)
found that a large proportion of tree roosts of lesser long-eared
bats were <5 m high, whereas most roosts of Gould’s wattled bats
were >5 m high. However, the latter species accounted for 91% of
bats present in boxes placed 4–6 m high in Organ Pipes National
Park (Irvine and Bender 1995; R. Bender, unpubl. data).
Use of artificial tree hollows by Australian birds and bats
The actual spacing of boxes may influence their frequency of
use. Nest boxes located too close together may be subject to
intraspecific competition and nesting failure of birds (see Krebs
1998). Among North American waterfowl, nest boxes placed at
high density can lead to high levels of conspecific brood
parasitism (multiple females depositing eggs into one clutch),
which can have adverse population effects (Eadie et al. 1998).
Studies of tree hollow use in white-tailed black-cockatoos
(Calyptorhynchus latirostris) and Major Mitchell’s cockatoos
(Cacatua leadbeateri) suggest that conspecific interactions lead
to a wide spacing of nest trees (Saunders et al. 1982).
Given the lack of proper testing of elements of hollow
design and placement, there is a great need for research on this
topic. The effective use of artificial hollows in research and
management will depend on sound knowledge of hollow
design and placement.
Hollow availability
The availability of natural hollows is commonly assumed to have
a strong influence on the frequency of use of artificial hollows
(e.g. Newton 1994). However, few studies in Australia have
considered this issue in detail with many studies having been
conducted where it was apparent that a dearth of natural hollows
was precluding use of an area (e.g. Irvine and Bender 1995) or
hindering reproduction (e.g. Orange-bellied Parrot Recovery
Team 2006). Three studies in Victoria (Ambrose 1982; Calder
et al. 1983; Menkhorst 1984) each installed boxes at several
locations where natural hollow abundance was either low or high.
In all studies boxes were used more frequently by Australian
owlet-nightjars where natural hollows were scarce. Whitethroated tree-creepers and crimson rosellas showed higher
use where natural hollows were scarce in two studies but no
difference in one study (crimson rosella) or higher use where
natural hollows were abundant (white-throated tree-creeper).
Ambrose (1982) found box use by barn owls (Tyto alba) to be
more frequent in forest with few hollows whereas boobook owls
(Ninox novaeseelandiae) showed no difference between sites
with few or many hollows. Menkhorst (1984) found that grey
shrike-thrushes (Colluricincla harmonica) only used boxes
where natural hollows were abundant.
Few studies have been conducted of bats in this context.
Golding (1979) found that 8% of boxes were used by the
lesser long-eared bat and Gould’s wattled bat where hollows
were abundant, compared to 3% where no hollows occurred and
43% where hollows were in low abundance. Ambrose (1982)
reported a single bat detection in a box whereas Menkhorst (1984)
reported none. Smith and Agnew (2002) found that Gould’s longeared bat used boxes frequently at two sites with no hollows and
there was little or no use where natural hollows were either in low
or high abundance. Goldingay et al. (2007) observed low use of
bat boxes across five locations that varied in abundance of natural
hollows including the two sites where Smith and Agnew (2002)
reported frequent use. For both birds and bats, it appears that site
factors (habitat, competitors) may have a strong influence on box
use that may mask a species’ response to natural hollow
abundance and that properly replicated studies are required to
better understand whether natural hollow abundance influences
use of artificial hollows. Due to the mobility of birds and bats it is
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87
likely that they can readily colonise areas devoid of natural
hollows if nest and roost boxes are installed.
Seasonal patterns related to breeding
Many species of hollow-using birds only use hollows for
breeding, so their breeding patterns will determine when nest
boxes are used. This may require frequent checking of artificial
hollows during the year to accurately document use as well as to
determine the adequacy of the artificial hollows provided for
breeding. The crimson rosella, white-throated tree-creeper
(Calder et al. 1983; Menkhorst 1984), grey shrike-thrush
(Menkhorst 1984), eastern rosella (Platycercus eximius) and
red-rumped parrot (Psephotus haematonotus) (Trainor 1995a)
used nest boxes in the spring and summer periods for breeding,
although nest site preparation (hole chewing and nest building) by
the crimson rosella (Calder et al. 1983; Pell and Tidemann 1997a;
Krebs 1998) and white-throated tree-creeper (Calder et al. 1983)
was observed during winter. In Victoria, crimson rosellas used
artificial hollows until chicks were fledged in mid-January, a
period of 56 days following egg laying (Golding 1979). Orangebellied parrots laid eggs between November and January, and had
an incubation period of 24 days and a nestling period of 35 days
(Orange-bellied Parrot Recovery Team 2006). Glossy blackcockatoos on Kangaroo Island laid eggs from late January to
late July, and had an incubation period of 30 days and a nestling
period of 90 days (Garnett et al. 1999). Red-tailed blackcockatoos laid eggs during September to December, and had
an incubation period of 30 days and a nestling period of 87 days
(Commonwealth of Australia 2006). Maned or wood ducks
(Chenonetta jubata) bred in nest boxes near Canberra between
July and December with a 34-day incubation period (Briggs
1991). The nocturnal Australian owlet-nightjar used nest boxes
primarily in spring and summer for breeding, but as a diurnal roost
site throughout the year (Calder et al. 1983; Menkhorst 1984;
Trainor 1995a). The white-throated tree-creeper was found to use
nest boxes for roosting all year except during summer (Ambrose
1982). The boobook owl and barn owl used nest boxes during
winter and spring, and less frequently during summer (Ambrose
1982). The above data indicate that seasonal patterns of use can be
highly variable among bird species and consequently the design
of monitoring programs will need to accommodate this.
Few data are available for bats but if artificial hollow use
mirrors natural hollow use then the requirements of some species
may differ during the breeding period from other times of the year,
when females aggregate into maternity colonies when breeding
(e.g. Churchill 1998). For example, females of the lesser longeared bat preferred enclosed hollows with fissure entrances in
very large-girth trees during breeding periods compared to when
not breeding (e.g. Lumsden et al. 2002a). Golding (1979) found
that individual or small groups of Gould’s wattled bat used roost
boxes throughout most of the year but female-dominated groups
(up to 39 bats) occurred in spring and for the lesser long-eared bat
(10 bats) in December in a few boxes. Smith and Agnew (2002)
observed small numbers (four to eight) of Gould’s long-eared bat
in several months but one aggregation of 16 females with young
in October. R. Bender (unpubl. data) observed a summer peak in
breeding by Gould’s wattled bats in boxes. The frequency with
which patterns of use mimic those within natural hollows by
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catering for non-breeding females as well as breeding
aggregations should indicate the effectiveness of artificial
hollows. Studies that involve a choice of box size and location
are needed to better understand the potentially varied seasonal
requirements of bats.
Temperature
The influence of temperature on hollow use will vary depending
on whether a species requires hollows throughout the year
(bats, hollow-roosting birds) or just during the breeding
season (hollow-breeding birds). Temperature within hollows
may be critical to the daily thermoregulation of individuals or
in promoting faster rates of growth by developing young
(e.g. Dawson et al. 2005). The temperature inside artificial
hollows can be an important factor in their use if hollows either
exceed or do not reach a preferred thermal zone. There may be a
seasonal element to this due to the poor insulative properties of
artificial hollows (Calder et al. 1983; Kerth et al. 2001), so they
may be ignored in winter when temperatures are low or ignored in
summer when temperatures are high (R. Goldingay, unpubl. obs).
This seasonal pattern is likely to vary latitudinally with climate and
may lead to variation in the preferred orientation of entrances to
nests (e.g. Burton 2007) or the preferred position of an artificial
hollow (full sun v. shade). Furthermore, elevational gradients in
temperature may also have an influence. Therefore, where we refer
to studies from different geographic locations below, we
acknowledge that some findings may be location specific.
In Australia, the influence of the thermal environment in
artificial hollows has not been well studied. Norman and
Riggert (1977) found that temperatures inside black plasticdrum nest boxes in southern Western Australia were an average
of 12C hotter than ambient. Temperatures in the boxes commonly
exceeded 35C in October but were not assessed later in the year
when they might have been much hotter. Menkhorst (1984)
suggested that reduced survival of white-throated tree-creeper
hatchlings in his study may have resulted from overheating
within nest boxes. Calder et al. (1983) have shown that aspect,
wall thickness, entrance position and box depth can all have
profound influences on temperatures within artificial hollows.
Overly high summer temperatures may not be such a problem
for hollow-breeding birds that breed in spring, but the young of
some species (e.g. crimson rosella, white-throated tree-creeper)
may not fledge until January–February (Calder et al. 1983). Some
bird species may choose warm hollows in spring due to the positive
influence that temperature can have on incubation and
nestling development (e.g. Ardia et al. 2006). Consistent
with this was the observation that Bechstein’s bats (Myotis
bechsteinii) in Germany preferred sun-exposed boxes during
lactation whereas shaded boxes were preferred pre-lactation
(Kerth et al. 2001).
Temperature is a critical issue for microbats in roost selection
(Boyles 2007) and therefore roost box selection. Many microbats
are quite small in size (<20 g) (Churchill 1998) so energy
conservation through thermoregulation will be critical to their
survival and the need for passive rewarming from daily torpor
may influence roost selection (e.g. Turbill et al. 2003a, 2003b;
Ruczynski 2006; Turbill 2006). This indicates that thermal
environments within roost boxes should have a strong
R. L. Goldingay and J. Stevens
influence on their occupation and frequency of use (Stebbings
and Walsh 1991). Indeed, this has been observed in microbats in
Europe and North America. Lourenço and Palmeirim (2004)
demonstrated that soprano pipistrelles (Pipistrellus pygmaeus) in
Portugal were tolerant of high temperatures (~40C) and preferred
black roost boxes over white or grey boxes. These boxes attained
greater diurnal temperatures and had temperature profiles more
similar to the roost cavities where many bats had been roosting.
Brittingham and Williams (2000) found that aspect had a
pronounced influence on temperatures within bat boxes in
Pennsylvania and that bats used boxes that experienced long
periods (7 h) of direct sun but not those with shorter periods
(5 h) of sun. In Germany, bats used sun-exposed bat boxes more
frequently than those that were shaded (Kerth et al. 2001). In
contrast, Neilson and Fenton (1994) recorded little use of four
types of bat box in New York State but noted that the mean
temperature in boxes was less than that in occupied roof cavities.
Thus, placement of boxes where suitable thermal ranges are not
experienced may result in avoidance by bats.
No studies in Australia have placed bat boxes with the explicit
purpose of creating hot microclimates inside. Menkhorst (1984)
placed boxes with a north or south entrance orientation but
detected no bats. Irvine and Bender (1995) placed bat boxes
across a range of aspects but most were predominantly in shaded
positions that they suggested may have contributed to a low
frequency of use by bats. Subsequent monitoring (R. Bender,
unpubl. data) has shown that the greatest use by Gould’s wattled
bat, the most frequent occupant, occurred in summer, which is
consistent with the hypothesis that the boxes do not have optimal
thermal environments. Smith and Agnew (2002) placed boxes
equally across the range of cardinal compass points but it is
unlikely that this had a consistent influence on direct sun due to the
cover of tree canopies at these sites (R. Goldingay, pers. obs).
Goldingay et al. (2007) placed bat boxes so that they would not
receive direct sun in the hottest months of the year because the
feathertail glider was the focus of their study. Lumsden et al.
(2002a) found that the highest proportion of roosts of the lesser
long-eared bat (tree hollows and some artificial structures) and
Gould’s wattled bat (all were tree hollows) had a north-north-east
aspect. A north aspect is likely to have a higher temperature profile
than other aspects but the selected aspects were not compared to
those of available sites. However, exfoliating bark of trees is
expected to have a random or even distribution and yet Turbill
et al. (2003a) found that the roosts under bark of the lesser longeared bat were significantly concentrated on the north-west aspect
of trees.
Future studies that target bats must position roost boxes with
consideration for the thermal requirements of bats. Studies should
investigate the influence of box design on temperature profiles.
Studies are also needed to investigate how the thermal
requirements of birds during nesting may influence hollow
choice. Furthermore, studies are needed to determine how
latitude and altitude may interact with temperature, and lead to
varied responses to artificial hollow placement.
Competitive interactions
Some hollow-using species may directly or indirectly exclude
others, which may reduce the effectiveness of artificial hollows
Use of artificial tree hollows by Australian birds and bats
for some target species. Competition may arise from native or
non-native species. The impacts of displacement by non-native
species is relatively well documented. The common myna (Pell
and Tidemann 1997a; Homan 1999; Harper et al. 2005b),
common starling (Sturnus vulgaris) (Ambrose 1982; Trainor
1995a; Pell and Tidemann 1997a, 1997b) and introduced
honeybee (Ambrose 1982; Trainor 1995b; Emison 1996; Pell
and Tidemann 1997a; Homan 1999; Harper et al. 2005b) are the
species of most concern, though the black rat could also be a
problem at some locations. These species have the potential to
deter native species from using nest boxes and reduce the number
of available hollows (Trainor 1995b; Pell and Tidemann 1997b;
Smith and Agnew 2002). The common myna was found to outcompete several species of native parrot and the common starling
can cause nest failure of native species (Ambrose 1982; Trainor
1995a; Pell and Tidemann 1997b; Gleeson 1999; Orange-bellied
Parrot Recovery Team 2006). Harper et al. (2005b) found that
mynas occupied 45 of 120 nest boxes in Melbourne over a
7-month period, and rebuilt their nests following removal.
Although competition for nest sites with the starling has led to
few declines among cavity-nesting birds in the USA (Koenig
2003), the longer-term effects of starlings and mynas on
Australian birds are unknown.
The invasion of both artificial and natural hollows by feral
honeybees can be a significant management issue (e.g. Trainor
1995b; Wood and Wallis 1998). The recovery plans of the
Kangaroo Island glossy black-cockatoo and orange-bellied
parrot have identified the need for regular maintenance to deter
and remove feral honeybees from artificial and natural hollows
(Mooney and Pedler 2005; Orange-bellied Parrot Recovery Team
2006). Feral honeybee invasion has been reported in many studies
of nest boxes (e.g. Wood and Wallis 1998; Harper et al. 2005b;
Goldingay et al. 2007) and needs to be documented in detail so its
incidence and treatment can be better understood. For example,
the use of carpet (Franks and Franks 2003) and insecticide strips
(Irvine and Bender 1995; Soderquist et al. 1996) inside nest boxes
have been trialled to prevent honeybee infestations but few details
have been reported.
Nest box design may mitigate the impact of pest species and
reduce the need for costly maintenance. Lumsden (1989) reported
that starlings did not use nest boxes painted white inside. In a
preliminary study by Homan (2000), the use of a baffle installed
on the front of nest boxes successfully excluded the common
myna without excluding native species. A slit entrance for bat
boxes has been suggested to exclude competition and predation
by introduced birds (Smith and Agnew 2002). Small nest box
volume may reduce hive building by honeybees (Goldingay et al.
2007). The influence of pest species is relevant to the success and
economic cost of artificial hollows in management, and further
investigation using an experimental approach is needed.
Some native hollow-using species will displace others. Boxes
installed in Australia for use by bats may be used by arboreal
mammals (Smith and Agnew 2002; Goldingay et al. 2007). Irvine
and Bender (1995) reported that 2 of 10 bat boxes had nests of
sugar gliders but over time all of these boxes had sugar gliders
present either often or occasionally (R. Bender and R. Irvine,
unpubl. data). Small entrance size will not overcome this problem
because feathertail gliders will use entrances of 1.5 cm and their
nests can completely fill a box (Fig. 7a). Goldingay et al. (2007)
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89
reported that 6 of 12 bat boxes at locations in south-east
Queensland had feathertail gliders or their nests and only four
boxes were used by bats. This may require a reliance on openbottom roost boxes for bats that preclude the construction of a leaf
nest inside (Fig. 7b, c).
Arboreal marsupials are likely to compete with birds as well
but the extent of this is not well documented. Menkhorst (1984)
provided circumstantial evidence that two bobucks (Trichosurus
caninus) discouraged the use of many nest boxes by other
mammals and birds. Common brushtail possums (Trichosurus
vulpecula) will kill nestlings present in artificial hollows. On
Kangaroo Island, this has required placing metal guards around
trees containing hollows used by glossy black-cockatoos to
prevent possums gaining access (Garnett et al. 1999). Sugar
gliders compete with orange-bellied parrots for nest sites and
may even kill incubating birds (Orange-bellied Parrot Recovery
Team 2006). This requires specific research to identify methods to
reduce this impact. For example, squirrel gliders and sugar gliders
may favour rear-entry nest boxes (the entrance occurs on the back
of the box; see Beyer and Goldingay 2006) and be less likely to use
other front-entry box types (with the entrance on the front or side
of the box) when a choice is provided (Goldingay et al. 2007;
R. Goldingay, unpubl. obs). Loeb and Hooper (1997) found that
the provision of nest boxes reduced the use of natural cavities
required by endangered red-cockaded woodpeckers (Picoides
borealis) by competing cavity users.
Crimson rosellas on Norfolk Island competed for nest boxes
with endangered Norfolk Island boobook owls (Olsen 1996) and
for natural hollows with endangered Norfolk Island green
parrots (Cyanoramphus novaezelandiae cookii) (Hill 2002).
Galahs (Cacatua roseicapilla) have been implicated in the
loss of eggs and little corellas (Cacatua pastinator)
implicated in the loss of nestlings of the glossy blackcockatoo (Garnett et al. 1999). Deployment of decoy nest
boxes may help to alleviate competition and provide easier
access to these species for control.
Emison (1996) recorded yellow-tailed black-cockatoos
(Calyptorhynchus funereus), long-billed corellas (Cacatua
tenuirostris), maned ducks and owls using artificial hollows
erected for red-tailed black-cockatoos. The expansion in the
geographic ranges of common hollow-nesting parrots may put
competitive pressure on other hollow-using native species.
Clearly, competition from non-target species can have a
substantial influence on the effectiveness of any artificial
hollow program. However, although the problem is well
acknowledged by many authors it remains poorly documented
and this will hamper attempts to reduce its impact. Therefore,
research must be conducted to understand the magnitude of the
problem and how it might be managed.
Artificial hollow applications
Artificial hollows have been used in a variety of ways for research
and management. Beyer and Goldingay (2006) recognised for
arboreal marsupials that nest boxes had three research
applications (detection of species, ecological studies,
investigation of nest-box design and placement) and three
management applications (threatened species recovery, species
introductions, strategic placement). These also apply to birds and
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Wildlife Research
R. L. Goldingay and J. Stevens
(a)
(b)
(c)
Fig. 7. (a) A feathertail glider nest completely filled this wedge-shaped bat box. (b) An open-bottom design
bat box (42 17 15 cm; front panel 32 cm). (c) Gould’s long-eared bats in an open-bottom box. Photos: (a, b)
R. Goldingay; (c) J. Lindsay.
bats. In addition, we also recognise ‘species establishment
through habitat enhancement’ and ‘hollow-bearing tree offset’.
These research and management applications are discussed in
detail below.
Research applications
Species detection
Nest boxes have been used as a survey tool to determine the
distribution and abundance of cryptic arboreal mammal species
(Beyer and Goldingay 2006). Using artificial hollows in this
way may not be of great importance for birds and bats, which
may be detected more readily using conventional survey
techniques, particularly sonar detection for bats (but see
Flaquer et al. 2007). Homan (1999) installed 12 nest boxes
to survey for small parrots but eight were quickly occupied by
common starlings and common mynas. However, the value of
nest and roost boxes is that they can be left in place for long
periods and may provide insights not readily gained from
periodic survey using other techniques. In Great Britain, the
placement of large numbers of bat boxes has produced
significant range extensions for some species (Stebbings and
Use of artificial tree hollows by Australian birds and bats
Walsh 1991). Artificial hollows could be deployed to identify
species that might respond to hollow provision, as a precursor to
a management application.
Ecological research
Nest boxes have been used extensively to investigate the
breeding biology and life histories of hollow-using birds in
North America and Europe (Koenig et al. 1992; Evans et al.
2002). In Australia, only a small number of published studies have
used artificial hollows to investigate the ecology of birds and none
have investigated the ecology of bats. Norman (1982) used nest
boxes to study egg laying and incubation in the chestnut teal.
Briggs (1991) used nest boxes to investigate intraspecific nest
parasitism in maned ducks. Pell and Tidemann (1997a) used nest
boxes to study factors that affect the breeding success of the
crimson rosella, and eastern rosella when in competition with
introduced hollow-using birds. Pell and Tidemann (1997b)
investigated the ecology of the common myna using nest
boxes. Nest boxes have allowed detailed investigation of
breeding biology, patterns of food allocation in broods, and
nestling growth and survival in the crimson rosella (Krebs
1998, 1999, 2001; Krebs et al. 1999, 2002; Krebs and
McGrath 2000). Nest box use by the glossy black-cockatoo
has allowed aspects of its breeding ecology to be described
(Garnett et al. 1999). Baltz and Clark (1999) used nest boxes
to investigate the influence of conspecifics on nest choice in
budgerigars (Melopsittacus undulates) in captivity. The above
studies highlight the value of nest boxes as a research tool,
in providing access to animals that may not be available any
other way.
Artificial hollow preferences
The design of artificial hollows can have a pronounced
influence on their frequency of use (see above). Despite this,
few preference studies have been conducted in Australia. Norman
and Riggert (1977) examined the use by waterfowl of eight nest
box types that differed in construction material and dimensions.
This revealed greater use of a thick wooden box (ammunition
box) but few of some types were installed and it appears that
a direct choice of different box types was not provided.
Menkhorst (1984) provides the first example of a properly
replicated choice experiment in which combinations of four
entrance sizes, three height placements and two aspect types
were used. He cautioned about the comparison of aspect because
boxes were placed on the east or west side of a tree and likely to
have received approximately equal exposure to the sun. He
identified a preference by crimson rosellas for entrance size
but sample sizes for owlet-nightjars, white-throated treecreepers and grey shrike-thrushes were too low to demonstrate
any choice. No bats were detected in this study.
R. Bender and R. Irvine (unpubl. data) explored the
relationship between entrance size and bat body size with a
small number of roost boxes. They suggested that Gould’s
wattled bat (10 g) preferred a slit entrance size >15 mm,
whereas smaller species, the large forest bat (7 g) and the
southern forest bat (Vespadelus regulus) (5 g), preferred slit
entrance sizes <12 mm. An entrance size <8 mm was unused
by bats. This suggests that competitive interactions may occur
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91
among bats for roost sites and boxes with different entrance sizes
may be required to avoid dominance by one bat species. It is clear
that many trials of different bat box designs are needed in
Australia. Furthermore, different bat species may prefer
different designs (other than just entrance size) so different bat
box designs should be field tested at multiple locations.
Lindenmayer et al. (2003) placed four box types (small with
small entrances (5 cm) or large with large entrances (10 cm),
installed high or low) on 24 plots across two locations to examine
box preferences by Leadbeater’s possum (Gymnobelideus
leadbeateri). The owlet-nightjar and crimson rosella were
recorded on only one occasion over a 3-year period. No other
birds or bats were recorded. In a study designed to target
Leadbeater’s possum, Harley (2004) installed 150 nest boxes
of various volumes, wall thicknesses and with 47–65 mm
diameter entrances but did not offer a direct choice of box
type. Over a 5-year period he observed a small amount of use
by owlet-nightjars, white-throated tree-creepers and one rosella
but no use by any bats (D. Harley, pers. comm.). This may have
been due to 76% of boxes having possums or their nest present
within. Goldingay et al. (2007) placed four box types on 45 plots
across five locations to examine box preferences by feathertail
gliders. They observed bats using only a wedge-shaped bat box
with a slit-entry at the bottom. Avoidance of another slit-entry
box may have been due to a lack of mesh inside the box to which
bats could attach. Two rear-entry box designs were probably
unsuitable for bats. Many further studies that offer a choice of
designs or design elements are needed to advance our
understanding of artificial hollow preferences.
Management applications
Threatened species recovery
Artificial hollows have been used in recovery programs for
several threatened bird species, but the deployment of roost boxes
for threatened bats has only just begun. Nest boxes have been used
in areas where natural hollows have been depleted. Information
on the use of nest boxes by threatened Australian parrots is largely
only documented in the recovery plans for these species and in a
cursory way. This needs to be properly reported in the peerreviewed literature for the usefulness of different artificial
hollows and management programs to be evaluated,
particularly so that this information can inform other projects.
Publication of research findings is a fundamental element of
conservation biology (Calver and King 1999).
Nest boxes have been in use for decades to supplement nesting
habitat of the critically endangered orange-bellied parrot, and
both wild and released captive-bred birds used nest boxes
(Orange-bellied Parrot Recovery Team 2006). The use of
nest boxes has become fundamental to assessing population
recovery by allowing access to individuals for banding and for
assessing reproductive success (Orange-bellied Parrot Recovery
Team 2006).
Artificial hollows have become important in the recovery of
the endangered Kangaroo Island glossy black-cockatoo. Over 80
artificial hollows made from PVC plumbing pipe (Fig. 3) have
been used to supplement natural nesting sites (Pedler 1996;
Mooney and Pedler 2005). Over several years, 25–35% of
successful breeding occurred in nest boxes, with breeding
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Wildlife Research
success similar (and in some years greater) to that in natural
hollows (Mooney and Pedler 2005). This program could inform
the use of artificial hollows in other recovery programs.
Artificial hollows consisting of hollows cut from fallen trees
have been used to supplement the breeding habitat of the southeastern red-tailed black-cockatoo. A program was trialled for this
subspecies and included erecting some nest boxes on 12 m-high
power poles (Emison 1996). Nest boxes were quickly occupied,
and over a 2-year period 30% of all artificial hollows were used
(Emison 1996). Approximately 60 artificial hollows, comprising
both PVC plumbing pipe and natural hollows cut from fallen trees
(Figs 2, 3) have been installed on power poles in the last 5 years
but only salvaged natural hollows (at least seven) were used
(R. Hill, pers. comm.). The recovery plan for this species has
recognised that although the availability of natural nest hollows is
not currently limiting the population, dead nest trees are
collapsing at a rate of 4–7% per year, and this is likely to be a
serious threat in years to come (Commonwealth of Australia
2007). The recovery plan recommends for the situation to be
monitored and the artificial hollow program expanded only if
significant nest tree losses are observed (Commonwealth of
Australia 2006). Artificial hollows made from fallen branches
were installed to supplement the breeding habitat of the turquoise
parrot (Neophema pulchella) in Chiltern Park, Victoria (Quin and
Baker-Gabb 1993). Two were used for nesting. Artificial hollows
made from salvaged tree hollows and erected in known breeding
areas have been used in breeding by Norfolk Island green parrots
(Hill 2002).
Nest boxes were installed in two studies in Tasmania to
target the endangered forty-spotted pardalote (Pardalotus
quadragintus) but were only successful in enabling the striated
pardalote (Pardalotus striatus) to breed (Milledge 1978;
Woinarski and Bulman 1985). Nest boxes are currently being
trialled as part of the recovery plan for this species (Threatened
Species Section 2006).
Roost boxes could play an important role in the recovery of
threatened bats but existing studies are inadequate to guide this
application. The recovery plan for the Christmas Island pipistrelle
(Pipistrellus murrayi) proposes to install roost boxes to
supplement the natural roosts of this species, which may
include beneath tree bark, under dead palm fronds, beneath
tree canopies, and in tree hollows (Schulz and Lumsden 2004).
The eastern false pipistrelle (Falsistrellus tasmaniensis), a
vulnerable species in New South Wales, has been recorded
using nest boxes in Victoria (Golding 1979; Ward 2000),
suggesting there is potential to use roost boxes in the recovery
of this species.
Existing studies demonstrate the usefulness of artificial
hollows to threatened species recovery. However, there is a
need for further research to improve and expand current use.
This application has potential for threatened bats but will depend
on a dramatic increase in our understanding of preferred roost
box designs.
Species introductions
We recognise this application as distinct from threatened
species recovery because it may not always involve such species
and the approach may be quite different. It involves installing
R. L. Goldingay and J. Stevens
artificial hollows at a location where a species is to be
introduced. The deployment of artificial hollows to introduce
a species at a site has been documented for just one species.
Several nest boxes were installed on Norfolk Island in 1987 for
the reintroduction of the endangered Norfolk Island boobook
owl (Ninox novaeseelandiae undulata) (Olsen 1996). Male
birds from the closest extant subspecies, the New Zealand
morepork (Ninox novaeseelandiae novaeseelandiae), were
translocated to the island. The nest boxes enabled successful
breeding to occur.
Species establishment through habitat enhancement
Because birds and bats are highly mobile it is quite possible
to attract and establish species at a location by the installation of
artificial hollows. This is as a form of habitat enhancement
where the decline or disappearance of hollow-using species is
recognised and artificial hollows have been installed to prevent
the permanent loss of biodiversity. Currently, the only
successful example of this management application is that by
Irvine and Bender (1995) for bats. Bat boxes were installed in
Organ Pipes National Park in Victoria in 1992 to facilitate the
establishment of bat populations within regenerating forest in
the park where habitat restoration had commenced in 1972
(Irvine and Bender 1995). Bat boxes were not occupied for
30 months, which is in stark contrast to within 3 months of
installation documented by Smith and Agnew (2002) in boxes in
Queensland. The number of bats in Organ Pipes National Park
increased from 15 per check in 1994–95 to >100 per check in
2004–05 from 34 boxes (R. Bender, unpubl. data). Gould’s
wattled bat comprised 91% of records but a small number of
large forest bats also consistently used the boxes. Boyd and
Stebbings (1989) reported a doubling over a 10-year period in a
population of brown long-eared bats (Plecotus auritus)
supported by roost boxes in managed forest in Great Britain.
These observations suggest that the local bat population in
Organ Pipes National Park increased in size over time so the
initial delay may have been due in part to a small local
population.
Harper et al. (2005b) installed nest boxes as habitat
enhancement (~3 per ha) for vertebrate fauna across remnant
vegetation within the urban and suburban landscape of
Melbourne. Although nest box design favoured large arboreal
mammals (large entrance and volume), a small number were used
by rainbow lorikeets (six), an eastern rosella (one) and a galah
(one). The provision of artificial hollows may also allow hollowusing species lost from urban areas to recolonise. Other studies
that install artificial hollows as habitat enhancement have been
planned and will target a range of birds and bats (B. Law and
R. Kavanagh, pers. comm.; R. Goldingay, unpubl. data).
This management application has considerable merit but is
currently hampered by the lack of understanding of the factors that
influence the use of artificial hollows (see above). Many trials are
needed to determine which hollow designs should be installed, the
number of each design, the most effective placement, and whether
particular species have benefited from such habitat enhancement.
Evidence that this application has been successful would be the
on-going use and breeding in artificial hollows (Petty et al. 1994),
rather than only sporadic use by species. Such trials should be
Use of artificial tree hollows by Australian birds and bats
conducted in an adaptive management framework so that changes
can be made as information accumulates on factors that improve
effectiveness. Monitoring will be fundamental to such studies and
needs to be continued for 2–5 years to provide the best insights.
Hollow-bearing tree offset
Another management application that has been used by land
managers (e.g. local government, road agencies, power
companies) is to install artificial hollows to compensate for
hollow-bearing trees lost during authorised clearing (Fig. 8).
This might occur either adjacent to the development site or
possibly away from the site if a landscape approach is taken to
managing hollow-using species. We stress that the potential value
of artificial hollows should not be used to justify the removal of
hollow-bearing trees. The merit of this application is that birds and
bats can be highly mobile and providing some replacement
hollows may assist some species as an interim solution while
other longer-term measures are devised. If such a management
application is used there needs to be monitoring over a 2–5-year
period to document the outcome. Currently, information is
lacking to demonstrate the value of such a use of artificial
hollows to hollow-using species so there is an obvious need
for research on this application.
Strategic placement
This management application emphasises the specific
locations where artificial hollows are placed. This might be a
specific location in a landscape such as within wildlife corridors or
it might be a location where a specific objective is to be achieved
such as to attract particular kinds of hollow-using species
(Beyer and Goldingay 2006). Although it will be concerned
with establishing species in an area it primarily differs from
‘species establishment’ because fewer hollows will be used,
their installation will be highly targeted and a more specific
objective will be stated.
In Europe there is increasing recognition of the value of nest
boxes to attract hole-nesting birds as a means of controlling insect
pests in forests and orchards (Mols and Visser 2002). Smith and
Fig. 8. Bat boxes installed on powerpoles through a powerline easement in
Brisbane. Some poles also have parrot boxes attached. These boxes were
installed after expansion of an existing powerline easement. Photo:
R. McLean.
Wildlife Research
93
Agnew (2002) hypothesised that small mammals, particularly
bats, could control insects in farm forests and therefore provide a
health benefit to the trees. They installed bat boxes in small
hardwood plantations in south-east Queensland and had success
in attracting a small number of bats to their sites but better success
with attracting feathertail gliders and squirrel gliders, which may
have discouraged greater use of the boxes by the bats. The work in
Organ Pipes National Park provides insight to the potential of
strategic placement with regard to bats. In 2004–05, >100 bats
were captured per round of box checks from 34 roost
boxes scattered through 5 ha of regrowth forest (R. Bender,
unpubl. data).
Artificial hollows are being trialled in northern Australia to
increase barn owl and masked owl numbers for the control of
rodent pests in sugarcane crops (Gibbons and Lindenmayer
2002). Studies on the strategic use of artificial hollows in
Australia are in their infancy. Further research is needed for
this application.
Conclusion
In many landscapes across Australia the collapse of hollowbearing trees has outpaced the recruitment of replacement
hollows and future shortages in this resource are inevitable
(Saunders 1979; Lindenmayer et al. 1990, 1997; Saunders
et al. 2003; Courtney and Debus 2006; Commonwealth of
Australia 2007; Beyer et al. 2008). The provision of artificial
hollows is likely to be the most appropriate interim solution to
this shortage but existing information on the use of artificial
hollows is too limited to enable this response to be effective and
progress in this field has been slow. Applying our criteria of
requiring more than a single record of artificial hollow use and
some details of the hollows, we found information was
documented for just 15 of 114 hollow-using bird species and
8 of 41 hollow-using microbat species.
The lack of information is most stark for microbat species.
The deployment of roost boxes for bats in Australia is clearly in
its infancy. Progress has in part been hampered by the relatively
infrequent use of artificial hollows by bats, which may be a
consequence of positioning roost boxes without regard for
creating suitable thermal environments. Selecting roosts to
enable passive rewarming from daily torpor may be quite
widespread among Australian tree-roosting bats (Turbill et al.
2003a, 2003b; Turbill 2006) and this will influence their
ability to use artificial hollows. Correct positioning of
artificial hollows poses a challenge because deployment in
forested habitats may preclude positioning boxes in highly
favourable microsites due to shading from canopy cover.
Some natural roosts of bats in mature forest may be located
high in trees (e.g. Herr and Klomp 1999; Lumsden et al. 2002a)
and have low canopy cover (e.g. Campbell et al. 2005) to
minimise shading. However, given that bats may commute over
long distances where tree hollows are scarce (e.g. Lunney et al.
1985, 1988; Lumsden et al. 2002b), it may be appropriate to
target edge and forest gap sites to place artificial hollows where
sun exposure will be optimal.
A key deficiency in deploying artificial hollows in
research and management of hollow-using birds and bats is in
understanding the most effective designs to use. Research
94
Wildlife Research
suggests that most species favour artificial hollows with
entrances just wide enough to enter. This enables avoidance of
larger competitors and possibly predators. Other elements of
design have not been properly investigated though guidance
can be provided by research on natural hollows, which show
that hollow depth is also important (e.g. Saunders et al. 1982;
Gibbons et al. 2002). Some bird species will use artificial hollows
made from salvaged tree limbs but it is not well documented
whether constructed hollows that emulate these have been tried
adequately. Bats may have different requirements during the year
(breeding v. non-breeding periods) and this might require the
provision of several designs. So few studies have been conducted
of bats using roost boxes in Australia that suitable designs
remain largely unknown. There is an obvious need for many
field experiments that compare different hollow designs or
which vary specific design elements for birds and bats. This
should lead to better knowledge of designs preferred by different
species.
Another aspect that requires further investigation is the
extent that competing species may pre-empt target species
from artificial hollows. Many studies have described potential
competition among hollow users but the seriousness of this to
management is not well documented. In some cases competitors
may favour a specific box design (e.g. arboreal mammals
that displace birds or bats) and exert less interference if their
hollow needs are catered for. This requires specific research
to address. Furthermore, there are various issues that relate to
the maintenance of artificial hollows, such as the occupation
by feral honeybees and the collapse of boxes (Beyer and
Goldingay 2006). This also must be addressed by research
with particular attention given to the cost of maintenance
(see also Harley 2006).
One aspect of artificial hollow placement that is not well
understood and in need of research is whether species show a
preference for the height at which boxes are placed on trees. This
is of considerable management relevance because this can have
cost and safety implications in effectively employing artificial
hollows. Available information suggests that heights >5 m are
rarely needed (Table 1). The only examples where hollows have
been placed very high is that of the Kangaroo Island glossy blackcockatoo with hollows at ~16 m above ground (Pedler 1996) and
the red-tailed black-cockatoo with hollows at 12 m above ground
(Emison 1996). It appears that artificial hollows have been placed
at an equivalent height to natural tree hollows (Pedler 1996). Such
hollows will require an enormous amount of time to check and
maintain. Given that red-tailed black-cockatoos have used tree
hollows at heights as low as 4.4 m (mean height 7 m) in Western
Australia (Saunders et al. 1982), it is possible that very high
hollows are not needed. Examples where bats select high roosts
(i.e. >10 m) may reflect their preference for suitable thermal
environments, which may be replicated by placing artificial
hollows in canopy gaps (see above).
Studies in Australia on the use of artificial hollows have lagged
behind those conducted in Europe and North America (e.g. Boyd
and Stebbings 1989; Newton 1994; Petty et al. 1994; Pogue and
Schnell 1994; Brittingham and Williams 2000; Kerth et al. 2001),
which is surprising given that a greater proportion of wildlife
species in Australia compared to other continents is dependent on
tree hollows for survival (see Saunders et al. 1982; Newton 1994;
R. L. Goldingay and J. Stevens
Gibbons and Lindenmayer 2002). The small number of
Australian studies currently available is insufficient to allow
many generalisations. Artificial hollows have enormous
potential to become an important management tool for hollowusing species in Australia but this can only be realised by
conducting many additional studies. This needs to occur
across a range of species and landscapes to maximise our
understanding of interactions between species and different
environments. Furthermore, species of Australian birds and
bats are likely to respond to similar factors in choosing
artificial hollows as taxonomically equivalent species overseas
(e.g. parrots, vespertilionid bats), so investigations can provide
independent tests of current hypotheses concerning influences on
use, or of management applications. This will allow greater
generalisation and can also lead to work conducted in
Australia informing research and management of hollow-using
species in other countries.
Acknowledgements
The comments of Brendan Taylor, Geoff Smith and two anonymous referees
helped improve this paper. This paper has been informed by several nest box
projects conducted in Brisbane and an artificial hollow project conducted at
Brunswick Heads, NSW. We thank Brisbane City Council for support of our
research in Brisbane and Abigroup for assistance with the project at Brunswick
Heads. Matthew Grimson and Geoff Smith are thanked for sharing ideas and
field work that have provided background to this paper. We thank Robert
Bender and Robert Irvine for their pioneering work with bat boxes in Organ
Pipes National Park.
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Manuscript received 1 May 2008, accepted 9 October 2008
http://www.publish.csiro.au/journals/wr